1. FIELD OF THE INVENTION
The present invention relates to an internal combustion engine intended to reduce the harmful components of exhaust gases by a simplified engine construction and, more particularly, to an improvement in a so-called "torch ignition type internal combustion engine" in which a lean air-fuel mixture in a main combustion chamber is ignited by a torch produced by the ignition of an air-fuel mixture charge in a trap chamber.
2. DESCRIPTION OF THE PRIOR ART
To operate an internal combustion engine with a lean air-fuel mixture is effective to reduce the harmful components of engine exhaust gases.
As is known, a torch ignition type internal combustion engine has a main combustion chamber and an auxiliary or trap chamber which is in communication with the main combustion chamber by a torch aperture or apertures and in which a set of electrodes of a spark plug is disposed. The trap chamber is supplied with a charge of an air-fuel mixture which is ignitable by a spark discharge of the spark plug to produce a torch which in turn ignites a charge of a lean air-fuel mixture in the main combustion chamber.
The torch ignition type internal combustion engine has its primary advantage that the production of harmful gases at the combustion stage of the air-fuel mixture is greatly minimized. The engine has a general characteristic that the engine performance is greatly influenced by the shape of the trap chamber, the size and orientation of the torch aperture or apertures and the positioning of the spark plug. The most important items of the engine performance in question are the ignitability and torch effect.
In the torch ignition type internal combustion engine, research has been conducted solely to decide the proper size of the torch aperture or apertures and the appropriate positioning of the spark plug in order to obtain improved ignitability (spark-ignition of mixture in the trap chamber) and torch effect (capability of ignition of the air-fuel mixture in the main chamber by torch or torches), but no serious attention has heretofore been given to the movement or flow of the air-fuel mixture within the trap chamber. It has been difficult to obtain an improved engine performance solely by the research on the torch aperture size and spark plug positioning. The torch aperture size and the spark plug positioning have been limited by other factors and have not been made optimum for the desired ignitability and torch effect.
The earlier copending application Ser. No. 610,889 referred to above discloses a torch ignition type internal combustion engine having a cylinder head which is provided with a trap chamber having suction and discharge apertures. On an intake stroke of the engine, i.e., when an intake valve is opened, a part of a charge of the air-fuel mixture is introduced through the suction aperture into the trap chamber and, at the same time, the residual gases are reliably discharged from the trap chamber through the discharge aperture into an associated main combustion chamber whereby the trap chamber is positively scavenged. The positive scavenging of the trap chamber results in an appreciable improvement in the ignitability of the air-fuel mixture when it is compressed in the trap chamber.
However, in order that an air-fuel mixture in the main combustion chamber may be reliably ignited and burnt, the mixture ignited in the trap chamber is required to produce a strong torch which is jetted from the trap chamber into the main chamber. This requirement has not been met to satisfactory extent.
In order to realize an optimum structure of a trap chamber for obtaining a torch sufficiently strong to reliably ignite and burn a mixture within a main combustion chamber, the inventors have examined the operation of a trap chamber of a conventional torch ignition type internal combustion engine. The engine used for the examination had a trap chamber formed in a cylinder head of the engine. The trap chamber was communicated with a main combustion engine through a single torch aperture. No intake valve was provided solely for the trap chamber. Thus, the trap chamber was of the type in which charges of air-fuel mixture were introduced through the torch aperture into the interior of the trap chamber. The trap chamber was not provided with any means which would cause or generate any positive movement of mixture charges within the trap chamber during not only the intake stroke but also during the compression stroke. In the internal combustion engine of the type and arrangement discussed, each charge of air-fuel mixture was introduced through the torch aperture into the trap chamber on each compression stroke. It was observed that residual gases were retained in the form of a stratum within the trap chamber at the end of the compression stroke and that the richness of the residual gases was increased as the distance within the trap chamber from the torch aperture toward the innermost end of the trap chamber was increased. This was because a charge of air-fuel mixture which entered the trap chamber through the torch aperture restrained the residual gases in the form of a stratum at the innermost part of the trap chamber as the mixture in the main combustion chamber was subjected to compression. In the case where a set of electrodes of a spark plug was positioned at the innermost end of the trap chamber, the existence of the stratum of residual gases greatly reduced the ignitability of the air-fuel mixture in the trap chamber particularly at a light or partial load operating condition of the engine. In addition, with the trap chamber having the plug electrodes positioned at the innermost end thereof, the front face of fire produced by ignition progressed toward the torch aperture and thus forced out a substantial part of the air-fuel mixture from the trap chamber into the main combustion chamber before the fire front face reached the torch aperture, so that the ignition of the air-fuel mixture in the trap chamber failed to provide a strong torch effect.
The discussed problem would be solved by disposing plug electrodes adjacent to the inlet portion of the trap chamber, namely, adjacent to the torch aperture of the trap chamber. Indeed, this positioning of plug electrodes has been put into practice and found to be effective to improve the ignitability in a torch ignition internal combustion engine of the type in which the main combustion chamber and the trap chamber are both supplied with charges of a homogeneous air-fuel mixture. With a stratified charge torch ignition internal combustion engine, however, a part of a lean air-fuel mixture flows from the main combustion chamber through a torch aperture or apertures into the trap chamber during the compression stroke. Thus, the lean air-fuel mixture is placed within the trap chamber adjacent to the torch aperture or apertures at the end of the compression stroke. Thus, the positioning of plug electrodes adjacent to the inlet or torch aperture or apertures does not result in an improvement in the ignitability of the air-fuel mixture within the trap chamber.
On the other hand, due to the positioning of plug electrodes adjacent to torch aperture or apertures, the fire initially produced by ignition of the air-fuel mixture adjacent to the plug electrodes first runs through the torch aperture or apertures into the main combustion chamber and, at the same time, progresses within the trap chamber toward the innermost end thereof with resultant increase in the duration and volume (or size) of the torch or torches. This, however, does not provide a sufficiently improved torch effect.
In anyway, therefore, the prior art trap chamber structure has failed to provide a sufficiently improved torch ignition of an air-fuel mixture in the main combustion chamber.